This invention relates to a process for the preparation of 2,3,5-trimethylhydroquinone (hereinafter referred to as TMHQ) by the catalytic oxidation of 2,3,5-and/or 2,3,6-trimethylphenol to 2,3,5-trimethyl-p-benzoquinone (hereinafter referred to as TMQ) followed by catalytic hydrogenation of the TMQ.
TMHQ has a rather broad utility in commerce, including application as a antioxidant and UV stabilizer in addition to its use as a starting material or intermediate in preparing pharmaceuticals. In particular, TMHQ finds special application as an intermediate for the production of alpha-tocopheral which is itself used in the production of vitamin E.
It is known that TMHQ can be prepared from 2,3,6-trimethylphenol by sequential oxidation and hydrogenation as described above. However, in general these two reactions require the use of different catalysts and usually the use of different solvents. Thus, from U.S. Pat. No. 3,658,852 it is known that 2,3,6-trimethylphenol may be oxidized to TMQ with oxygen in the presence of a cobalt chelate complex such as bis(salicylideneethylenediimine)cobalt(II) as catalyst and a substituted amide, for example dimethylformamide, as solvent. In contrast, U.S. Pat. No. 3,839,468 teaches a process for catalytic hydrogenation of TMQ to TMHQ which employs a palladium catalyst and an aliphatic ketone solvent. This second patent also discusses other, apparently less successful, prior art attempts to conduct the catalytic hydrogenation reaction in solvents such as alcohols, hydrocarbons, ethers and carboxylic acids and gives comparative examples using solvent such as tertiary butanol and diisopropyl ether.
While the preparative scheme described above may be suitable for laboratory scale synthesis of TMHQ it suffers certain disadvantages when applied on a commercial scale. Specifically the change from one catalyst/solvent system to another such system can give rise to problems when these two reactions are to be carried out successively on a large scale. That is, both the catalyst and solvent must be removed completely from the reaction product of the initial oxidation reaction before the catalyst and solvent for the second reaction can be added to the system. Thus, unless considerable care and associated expenses is involved in the initial catalyst and solvent removal, the initial reaction product will likely be contaminated with residual catalyst and/or solvent which may adversely effect the subsequent hydrogenation reaction. The present invention provides a method whereby the two reactions may be conveniently carried out in sequence to afford high yields of TMHQ.